Inverter apparatus

ABSTRACT

Induction voltage command Em* is obtained from inverter&#39;s primary frequency command ω 1 * and torque boost voltage commander produces torque boost voltage command ΔVz* in accordance with ω 1 * while integrator produces reference phase command θd*. uvw/dq converter detects motor excitation current Id (equivalent of no-load current). Next, deviation of excitation current limitation level command Idmax* and detected Id is inputted to limiter processing unit to produce torque boost voltage compensation value ΔVc for varying ΔVz* so that Id is smaller than or equal to Idmax*. Inverted ΔVz* is set up as a lower limiter value of the limiter processing unit. Next, ΔVc and ΔVz* are added to produce final compensated torque boost voltage command ΔVt* and ΔVt* and Em* are added to produce q-axis voltage command Vq* of the inverter output voltage.

BACKGROUND OF THE INVENTION

The present invention relates to an inverter apparatus for controllingthe speed of an induction motor variably.

As a method of controlling an inverter for driving the induction motorso that the induction motor is operated at variable speed, there isknown a V/f fixed control method of controlling an output voltage (V1)of the inverter in proportion to a primary frequency (f1) of theinverter. This method has a problem that when a load is applied, aninduced voltage (Em) of the induction motor is reduced because of avoltage drop across a primary resistance (r1) of the induction motor, sothat a magnetic flux of the induction motor is made small andaccordingly a maximum torque is reduced.

In order to increase a torque in a low and medium speed area, a generalinverter includes torque boost function. When a large start torque isrequired, a boost voltage is set up to a high voltage in a low speedarea and the boost voltage is added to a V/f fixed voltage command(induced voltage command Em*) to produce an output voltage command ofthe inverter. However, when the boost voltage is increased,over-excitation occurs in no load. When the over-excitation occurs, themagnetic flux of the induction motor is saturated and accordingly anexcitation reactance is reduced to thereby increase an excitationcurrent. Consequently, the temperature of the induction motor rises orthe current of the inverter is increased excessively, so that there isthe possibility that over-current protection function or over-loadprotection function is operated to be tripped.

A method of suppressing the over-excitation is described in, forexample, JP-A-7-163188. In this method, a command for setting up afrequency to zero is issued before start of operation and a DC currentis supplied to the induction motor. An output voltage of the inverter atthe time that a current of U-phase becomes equal to an equivalent of adesign value of the excitation current is set up as a torque boostvoltage ΔVz0 at the time that the frequency is 0 Hz.

SUMMARY OF THE INVENTION

In the above method, since a torque boost voltage is set up so that thecurrent in no load is equal to a rated excitation current (design valueof excitation current), no over-excitation occurs. In this case,however, the voltage drop across the primary resistance is increasedwhen the induction motor is loaded. As a result, there is a problem thatthe induced voltage (magnetic flux of motor) is reduced to therebydecrease an output torque. In this manner, when the torque boost voltageis made high, the torque is increased, while over-excitation occurs whenthe load is light. Conversely, when the torque boost voltage is madelow, the over-excitation does not occur, while there is an antitheticproblem that the torque is not increased.

It is an object of the present invention to provide an inverterapparatus suitable for prevention of over-excitation even when a torqueboost voltage is set up to be high in order to obtain a large starttorque in a general inverter.

In order to achieve the above object, the inverter apparatus accordingto an aspect of the present invention comprises detection means fordetecting an excitation current of the induction motor, setting meansfor setting a limitation level of the excitation current, torque boostvoltage command means for producing a torque boost voltage command inresponse to a frequency command of the inverter apparatus, and torqueboost voltage compensation means for changing the torque boost voltagecommand so that the detected excitation current value is smaller than orequal to the excitation current limitation level.

The torque boost voltage compensation means includes limiter processingmeans and inverts the torque boost voltage command. The inverted torqueboost voltage command is limiter-processed as a lower limiter value ofthe limiter processing means to produce a compensation value of thetorque boost voltage command.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating an inverterapparatus according to an embodiment of the present invention;

FIG. 2 is a graph showing a characteristic of a q-axis voltage commandVq* shown in FIG. 1;

FIGS. 3A and 3B are circuit diagrams illustrating a T-type equivalentcircuit and an equivalent circuit at a low frequency of an inductionmotor, respectively;

FIGS. 4A and 4B are vector diagrams illustrating output voltages andcurrents of the inverter in no load and heavy load in the presentinvention, respectively;

FIGS. 5A and 5B are graphs showing an output voltage characteristic andan output current characteristic of an inverter apparatus when a torqueboost voltage is varied in no load state in the control of the presentinvention;

FIG. 6 is a block diagram schematically illustrating an inverterapparatus according to another embodiment of the present invention;

FIG. 7 is a block diagram illustrating an Id (excitation current)detector shown in FIG. 6 in detail; and

FIG. 8 is a graph showing a relation of three-phase voltage commandsVu*, Vw* and Vv* and sections I to VI.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention are now described with reference tothe accompanying drawings. In the drawings, like elements are designatedby like reference numerals.

FIG. 1 illustrates a control block of an inverter apparatus according toan embodiment of the present invention for controlling the speed of aninduction motor variably.

An AC power from an AC power supply 1 is converted into a DC power bymeans of a rectification circuit 2 and a smoothing capacitor 3. The DCpower is converted into an AC variable voltage having a variablefrequency by means of an inverter unit 4 to drive an induction motor 550that the induction motor is operated at variable speed. An outputfrequency and an output voltage of the inverter unit 4 are controlled byan inverter control circuit including, for example, a pate circuit 6, aV/F gain 7, a torque boost voltage commander 8, an integrator 9, a uw/dqconverter 11, a PI controller 12, a limiter processing unit 13, aprimary resistance 14, a dq/uw converter 15, and a gate supportgenerator 16, as shown in FIG. 1.

In the inverter control circuit of the embodiment, a primary frequencycommand ω1* of the inverter unit 4 is multiplied by a V/f gain 7 toproduce an induced voltage command Em*. Further, a torque boost voltagecommander 8 produces a torque boost voltage command ΔVz* in accordancewith the primary frequency command ω1*. In this connection, ΔVz0 is atorque boost voltage set value. Then, the primary frequency command ω1*is integrated by an integrator 9 to produce a reference phase commandθd* which is a phase reference of the output voltage of the inverterunit 4. Further, a uvw/dq converter 11 makes calculation of the equation(1) on the basis of output currents iu and iw of a motor currentdetector 10 and the reference phase command θd* to detect an excitationcurrent Id (equivalent of no-load current) of the inductor motor 5.

 Iv=−(iu+iw)

Id=iu·cos θd*+iv·cos(θd*+2π/3)+iw·cos(θd*+4π/3)  (1)

Next, a deviation of the an excitation current limitation level commandIdmax* and the detected excitation current value Id is amplified by a PI(proportion and integration) controller 12 and an output of the PIcontroller 12 is supplied to a limiter processing unit 13. The limiterprocessing unit 13 processes the output of the PI controller 12 toproduce a torque boost voltage compensation value ΔVc. Here, the torqueboost voltage command ΔVz* is inverted by an inverter and the invertedtorque boost voltage command ΔVz* is used as a lower limiter value ofthe limiter processing unit 13. The lower limiter value is varied inaccordance with the primary frequency command ω1* of the inverter unit4. Further, ΔVc and ΔVz* are added to produce a final compensated torqueboost voltage command ΔVt*. Then, ΔVt* is added to the induced voltagecommand Em* to produce a q-axis voltage command Vq* of the inverteroutput voltage. On the other hand, a d-axis voltage command Vd* of theinverter output voltage is calculated by multiplying a rated excitationcurrent command Id* by an equivalent of a primary resistance r1 of themotor in a primary resistance constant circuit 14. Then, a dq/uvwconverter 15 is supplied with the rotating coordinate axis componentsVd* and Vq* of the inverter output voltage command and producesthree-phase voltage commands Vu*, Vv* and Vw* for the fixed coordinateaxis. This calculation is expressed by the equation (2).

 Vu*=Vd*·cos θd*−Vq*·sin θd*

Vw*=−Vu*/2−{square root over ( )}3(Vd*·sin θd*+Vq*·cos θd*)/2

Vv*=−(Vu*+Vw*)  (2)

Further, a gate signal generator 16 prepares PWM gate signals on thebasis of the three-phase voltage commands Vu*, Vv* and Vw* to supply thePMW gate signals to a gate circuit 6.

FIG. 2 shows a range of the q-axis voltage command Vq* which is therotating coordinate axis component of the inverter output voltagecommand.

For example, the magnitude of Vq* at the primary frequency commandω1*=ω1x is a value at point a of the induced voltage command Em* whenthe load is zero and since Vq* is small, the over-excitation can beprevented. On the other hand, when the load is heavy, the magnitude ofVq* at the primary frequency command is a value at point b of Em*+ΔVz*and since Vq* is large, large torque is obtained. Further, when the loadis intermediate thereof, it is a value at point c of Em*+ΔVz*−ΔVc, forexample. That is, since the value of Em*+ΔVz* at point b is compensatedby ΔVc, it is the value at point c. In this manner, the torque boostvoltage compensation value ΔVc is varied within the range from the pointb to the point a in accordance with the load. That is, the torque boostvoltage compensation value ΔVc is varied between upper and lower brokenlines.

Incidentally, when there is no limitation control of the excitationcurrent, the torque boost voltage compensation value ΔVc is 0 andaccordingly the upper broken line becomes Vq*. In the case of Vq*,over-excitation occurs when the load is light at low speed area. In theembodiment, the limitation control of the excitation current is made sothat when the load is light the torque boost voltage compensation valueΔVc is varied between the upper and lower broken lines to reduce Vq* sothat over-excitation does not occur.

An operation of the embodiment is now described concretely.

First, when the load is lightened or lowered, the detected excitationcurrent value Id is increased and when the limitation level Idmax* isexceeded, the PI controller 12 is supplied with a negative value. Atthis time, the torque boost voltage compensation value ΔVc becomes alsonegative. At this time, ΔVc is functioned to subtract the torque boostvoltage command ΔVz* so that the final compensated torque boost voltagecommand ΔVt* is controlled to make the excitation current Id equal tothe excitation current limitation level Idmax* (Id=Idmax). Then, whenthe load is heavy, the excitation current Id is smaller than theexcitation current limitation level Idmax* (Id<Idmax*) and accordinglythe compensation value ΔVc is increased from the negative value to be avalue of −ΔVz to 0. Consequently, the final torque boost voltage commandΔVt* becomes 0 to ΔVz when the load is heavy.

As described above, when the load is light, the final compensated torqueboost voltage command ΔVt* is reduced so that the excitation current Idis equal to the excitation current limitation level Idmax* (Id=Idmax*)and when the load is heavy, the final compensated torque boost voltagecommand ΔVt* is increased conversely. Since the compensation value ΔVcis varied within the range of the boost voltage command ΔVz* by means ofthe limiter processing unit 13, the final compensated torque boostvoltage command ΔVt* is operated within the range of 0≦ΔVt* ≦ΔVz* tothereby prevent excessive compensation.

Operation of the embodiment is now described with reference to anapproximate equivalent circuit and voltage and current vector diagramsof the induction motor 5.

FIG. 3A illustrates a T-type equivalent circuit. r1 and r2 representprimary and secondary resistances, x2, x2 and xm represent primary andsecondary leakage reactances and excitation reactance, respectively.Further, s represents slip. In the low-frequency area in which thetorque boost control is required, x1≦r1 and x2≦r2/s. Accordingly, in thelow-frequency area, the induction motor 5 can be approximated by theequivalent circuit of FIG. 3B.

FIGS. 4A and 4B show voltage and current vector diagrams of theinduction motor 5 in no load and heavy load using the approximateequivalent circuit.

In no load, since the slip s=0 and the secondary current I2=0, theequivalent circuit becomes a series circuit of r1 and xm and the primarycurrent I1 is equal to the excitation current Im (I1=Im). Accordingly,the primary voltage vector V1 is given by the equation (3), where jrepresents the imaginary number.

 V1=Im(r 1+jxm)  (3)

Further, when the d-axis voltage command Vd* is given by Id*·r1 and theq-axis voltage command Vq* is given by jIm·xm, the excitation current Im(no-load current) is approximately equal to Id shown by the equation (1)and the excitation current Im can be detected by Id. Id* represents therated excitation current (no-load current) command.

The broken line of FIG. 4A shows the case where there is no limitationcontrol of the excitation current and the primary voltage V1′ is high.At this time, since the primary voltage V1′ is high, the excitationcurrent Id (Id≈Im′) is larger than the limitation level Idmax*, so thatover-excitation occurs. The solid line of FIG. 4A shows the case wherethe limitation control of the excitation current of the embodiment iseffective. In this case, since the voltage V1 is reduced so thatId≦Idmax*, the no-load current Id (Id=Im) is approximately equal toIdmax*, so that over-excitation is prevented.

Next, operation in the heavy load is described. In this case, theequivalent circuit is as shown in FIG. 3B and the secondary current I2is increased while the power-factor angle φ (angle between V1 vector andI1 vector) is decreased. At this time, the induced voltage Em is greatlyreduced as compared with V1 due to a voltage drop across the primaryresistance r1 and Im=Id<Idmax*. At this time, since Id<Idmax*, thetorque boost voltage compensation value ΔVc becomes 0 (ΔVc=0).Consequently, since the torque boost voltage command ΔVz* is added as itis, the inverter output voltage is increased so that reduction of Em iscompensated and the large start torque is obtained.

FIGS. 5A and 5B show characteristics of the inverter output current I1and the inverter output voltage V1 in the case where the torque boostvoltage set value ΔVz0 is increased gradually when the output frequencycommand of the inverter unit 4 is fixed to a low frequency and theinverter unit 4 is operated in no load in control of the embodiment.

When there is no limitation control of the excitation current, theoutput current I1 and the output voltage V1 are increased with increaseof ΔVz0 as shown by broken line. On the other hand, when the embodimentis applied (when the limitation control of the excitation current iseffective), the output current I1 is not increased after the time thatthe output current I1 approximately reaches Idmax* (I1≈Idmax*) as shownby solid line. Consequently, the excitation current (no-load current) islimited and accordingly over-excitation does not occur. Further, asshown by solid line of FIG. 5B, the inverter output voltage V1 is notalso increased and accordingly over-excitation does not occur.

FIG. 6 schematically illustrates another embodiment of the presentinvention. This embodiment is different from the embodiment of FIG. 1 inthat the excitation current Id is detected from the inverter inputcurrent idc. The excitation current Id is detected on the basis of anoutput signal idc of an inverter input current detector 17, the gatesignal of the inverter and the reference phase command θd* in anexcitation current detector 18.

FIG. 7 illustrates a detail configuration of the excitation currentdetector 18. The excitation current detector 18 is composed of asample-and-hold signal preparation circuit 19, a sample-and-holdcircuits 20 a and 20 b and an Id arithmetic unit 21. The sample-and-holdsignal preparation circuit 19 produces a sample-and-hold signals SHa andSHb on the basis of PWM gate signals by means of logical AND circuits 22and logical OR circuits 23 as shown in FIG. 7. In the circuit of FIG. 7,the inverter input current idc is sampled and held in the switching modethat only one phase gate signal of three-phase gate signals is turned onto be outputted as an ia signal. Further, in the switching mode thatonly two phases are turned on, idc is sampled and held to be outputtedas an ib signal. Then, the Id arithmetic unit 21 performs calculation ofthe equation (4) to produce Id. FIG. 8 shows a relation of waveforms ofthree-phase voltage commands Vu*, Vw* and Vv* and sections I to VI.

In the section I,

Vu*≧Vw*>Vv*,

iα=−ia, iβ=(ia−2ib)/{square root over ( )}3,

Id=iα·cos(θd*−2π/3)+iβ·sin(θd*−2π/3)

In the section II,

Vu*≧Vv*>Vw*,

iα=ib, iβ=(2ia−ib)/{square root over ( )}3,

Id=iα·cos θd*+iβ·sin θd*

In the section III,

Vv*≧Vu*>Vw*,

iα=−ia, iβ=(ia−2ib)/{square root over ( )}3,

Id=iα·cos(θd*−4π/3)+iβ·sin(θd*−4π/3)

In the section IV,

Vv*≧Vw*>Vu*,

iα=ib, iβ=(2ib−ia)/{square root over ( )}3,

Id=iα·cos(θd*−2π/3)+iβ·sin(θd*−2π/3)

In the section V,

Vw*≧Vv*>Vu*,

iα=−ia, iβ=(ia−2ib)/{square root over ( )}3,

Id=iα·cos θd*+iβ·sin θd*

In the section VI,

Vw*≧Vu*>Vv*,

iα=ib, iβ=(2ib−ia)/{square root over ( )}3,

Id=iα·cos(θd*−4π/3)+iβ·sin(θd*−4π/3)  (4)

Discrimination of the 60-degree sections I to VI is made on the basis ofthe magnitude of the three-phase voltage commands produced by the dq/uvwconverter 15. Further, the discrimination of the 60-degree sections I toVI can be also made similarly by using the voltage command phase θd*.The system for detecting the excitation current Id from the DC currentidc is described in JP-A-2001-314090 in detail.

In the embodiment of FIG. 6, only one inverter input current detector 17may be used to detect the excitation current Id and the motor currentdetector (for two phases) as shown in the embodiment of FIG. 1 is notrequired, so that the inverter apparatus can be configuredinexpensively.

As described above, according to the embodiment, since the torque boostvoltage can be adjusted automatically so that the excitation current issmaller than or equal to the limitation level even when the torque boostvoltage is set up to be large in the torque boost control of theinverter, over-excitation does not occur in light load. Furthermore,since the torque boost voltage can be set up to be high, large starttorque can be obtained even in heavy load.

Further, since over-excitation does not occur even when the torque boostvoltage is set up to be high, it is not necessary to adjust the torqueboost voltage in accordance with the magnitude of load. Accordingly,adjustment is not required and handling is good.

It should be further understood by those skilled in the art that theforegoing description has been made on embodiments of the invention andthat various changes and modifications may be made in the inventionwithout departing from the spirit of the invention and the scope of theappended claims.

What is claimed is:
 1. An inverter apparatus for converting a DC powerfrom an input AC power to an output AC power having a variable frequencyand a variable electric power to drive an induction motor at a variablespeed, comprising: a rectifying unit for converting an input AC power toa DC power, a filter capacitor for smoothing the DC power outputted fromsaid rectifying unit, an inverter unit having an input connected acrosssaid filter capacitor, a motor current detector for detecting a motorcurrent outputted from said inverter unit and a gate circuit for drivingsaid inverter unit; an excitation detection unit for detecting anexcitation current of said induction motor from an output of said motorcurrent detector and a reference phase command; a setting unit forsetting a limitation level of said excitation current; a torque boostvoltage command unit for producing a torque boost voltage commandaccording to an inverter frequency command; and a torque boost voltagecompensation unit for changing said torque boost voltage command so thatthe detected excitation current value is smaller than or equal to saidlimitation level.
 2. An inverter apparatus according to claim 1, whereinsaid torque boost voltage compensation unit includes a limiterprocessing unit, and inverts said torque boost voltage command, which islimiter-processed as a lower limiter value of said limiter processingunit to produce a compensation value of said torque boost voltagecommand.
 3. An inverter apparatus according to claim 1, wherein ano-load motor current is limited substantially to an excitation currentlimitation level when said torque boost voltage command is increasedgradually in the state that said induction motor is being operated in noload.
 4. An inverter apparatus according to claim 1, wherein an inverteroutput voltage is controlled to be substantially constant after the timewhen a no-load motor current reaches substantially an excitation currentlimitation level when said torque boost voltage command is increasedgradually in the state that said induction motor is being operated in noload.
 5. An inverter apparatus according to claim 2, wherein a no-loadmotor current is limited substantially to an excitation currentlimitation level when said torque boost voltage command is increasedgradually in the state that said induction motor is being operated in noload.
 6. An inverter apparatus according to claim 2, wherein an inverteroutput voltage is controlled to be substantially constant after the timewhen a no-load motor current reaches substantially an excitation currentlimitation level when said torque boost voltage command is increasedgradually in the state that said induction motor is being operated in noload.
 7. An inverter apparatus for converting a DC power from an inputAC power to an output AC power having a variable frequency and avariable electric power to drive an induction motor at a variable speed,comprising: rectifying means for converting an input AC power to a DCpower, filter means for smoothing the DC power outputted from saidrectifying means, inverter means having an input connected across saidfilter means, motor current detection means for detecting a motorcurrent outputted from said inverter means, and gate drive means fordriving said inverter means; excitation current detection means fordetecting an excitation current of said induction motor from an outputof said motor detection means and a reference phase command; settingmeans for setting a limitation level of said excitation current; torqueboost voltage command means for producing a torque boost voltage commandaccording to an inverter frequency command; and torque boost voltagecompensation means for changing said torque boost voltage command sothat the detected excitation current value is smaller than or equal tosaid limitation level.
 8. An inverter apparatus according to claim 7,wherein said torque boost voltage compensation means includes limiterprocessing means, and inverts said torque boost voltage command, whichis limiter-processed as a lower limiter value of said limiter processingmeans to produce a compensation value of said torque boost voltagecommand.
 9. An inverter apparatus according to claim 7, wherein ano-load motor current is limited substantially to an excitation currentlimitation level when said torque boost voltage command is increasedgradually in the state that said induction motor is being operated in noload.
 10. An inverter apparatus according to claim 7, wherein aninverter output voltage is controlled to be substantially constant afterthe time when a no-load motor current reaches substantially anexcitation current limitation level when said torque boost voltagecommand is increased gradually in the state that said induction motor isbeing operated in no load.
 11. An inverter apparatus according to claim8, wherein a no-load motor current is limited substantially to anexcitation current limitation level when said torque boost voltagecommand is increased gradually in the state that said induction motor isbeing operated in no load.
 12. An inverter apparatus according to claim8, wherein an inverter output voltage is controlled to be substantiallyconstant after the time when a no-load motor current reachessubstantially an excitation current limitation level when said torqueboost voltage command is increased gradually in the state that saidinduction motor is being operated in no load.
 13. An inverter apparatus,comprising: a conversion unit for converting an input AC power to a DCpower; an inverter unit for converting the DC power into an output ACpower having a variable frequency and a variable electric power to drivean inductor motor at a variable speed; an excitation current detectionunit for detecting an excitation current of said induction motor; and aninverter control circuit arranged to control the variable frequency andthe variable electric power of the AC power outputted from the inverterunit, said inverter control circuit comprising: a setting unit forsetting a limitation level of the excitation current; a torque boostvoltage command unit for producing a torque boost voltage commandaccording to an inverter frequency command; and a torque boost voltagecompensation unit for compensating the torque boost voltage command,when the excitation current is equal to or larger than a predeterminedvalue, and for generating a compensated torque boost voltage commandcontrolled so that the excitation current value detected is smaller thanor equal to said limitation level of the excitation current.
 14. Aninverter apparatus according to claim 13, wherein the torque boostvoltage compensation unit includes a limiter processing unit arranged toprocess an invert of the torque boost voltage command as a lower limitervalue so as to generate a compensated torque boost voltage command. 15.An inverter apparatus according to claim 13, wherein a no-load motorcurrent is limited substantially to an excitation current limitationlevel when the torque boost voltage command is increased gradually inthe state that the induction motor is being operated in no load.
 16. Aninverter apparatus according to claim 13, wherein an inverter outputvoltage is controlled to be substantially constant after the time when ano-load motor current reaches substantially an excitation currentlimitation level when the torque boost voltage command is increasedgradually in the state that the induction motor is being operated in noload.
 17. An inverter apparatus according to claim 13, wherein theinverter control circuit further comprises: an integrator arrange tointegrate the inverter frequency command to produce a reference phasecommand; a three-phase converter arranged to generate three-phasevoltage commands for a fixed coordinate axis according to coordinateaxis components of the compensated torque boost voltage command, anInduced voltage command and the reference phase command; a gate signalgenerator arranged to prepare gate signals according to the three-phasevoltage commands; and a gate circuit arranged to drive the inverter unitaccording to the gate signals.
 18. An inverter apparatus according toclaim 17, wherein the excitation current detection unit comprises: asignal preparation circuit including a series of logical AND gates andlogical OR gates arranged to prepare the gate signals; sample-and-holdcircuits arranged to sample and hold the gate signals prepared from thesignal preparation circuit; and an arithmetic circuit arranged toperform a predetermined calculation of sampled signals on the basis ofthe three-phase voltage commands.